Measuring higher voltages than rated oscilloscope capacity?

[shrinky]

I have a cheap oscilloscope that is rated for 10V input.

I want to measure the quality of the output of my power supply due to a belief the DC filtering circuit isn't working well and producing harmonics in the signal (ripples). The output voltage is at a higher voltage coming out of a switching mode power supply unit with output of 40V.

As a quick and dirty solution, I was wondering how I measure it using my oscilloscope?

I am thinking putting two resistive loads in series and then applying the DC load across them, and then measuring just one resistor. Say I make 1MOhm and 5MOhm in series, then I measure across the 1MOhm resistor using my 10V rated oscilloscope.

Does this work? Any better suggestions?

My primary concern is not damaging my oscilloscope or powersupply.

 

[torquil]

I have a cheap oscilloscope that is rated for 10V input.

I want to measure the quality of the output of my power supply due to a belief the DC filtering circuit isn't working well and producing harmonics in the signal (ripples). The output voltage is at a higher voltage coming out of a switching mode power supply unit with output of 40V.

As a quick and dirty solution, I was wondering how I measure it using my oscilloscope?

I am thinking putting two resistive loads in series and then applying the DC load across them, and then measuring just one resistor. Say I make 1MOhm and 5MOhm in series, then I measure across the 1MOhm resistor using my 10V rated oscilloscope.

Does this work? Any better suggestions?

Yes, you have the right idea. It does work, and it is called a voltage divider. I've done this myself in order to get a low-voltage signal into the line in on my computer, starting from a very high voltage signal in a tube amplifier. Needless to say, I used a much higher ratio of resistors than 1:5 as in your case. Actually, I would suggest that you try a higher ratio to eliminate all risk of blowing the oscilloscope input. E.g. at least 1:10.

Note that if the input impedance of you oscilloscop is not much larger than 1 MOhm, it will affect the voltage divider so the ratio of input and output voltages will no longer be 1:5. Worst case, it will make the measured signal a bit weaker than it really is, so there is no danger that this will case damage to the oscilloscope. You probably don't need to go as high as 1MOhm and 5MOhm though. They only need to be large compared to the output impedance of your power supply, which is typically very low. I'd use e.g. 1kOhm and 5kOhm. Make sure the resistors can handle the power dissipation, though.

Btw, when I did this with my tube amp and computer, I made myself very sure that the ground on the amp side was connected to the ground on the computer side. And I would only recommend doing it with a cheap computer... Not sure if this is relevant for real oscilloscopes.

You might also put a simple high-pass filter between the power supply and the oscilloscope, in order to block DC, but pass all the frequencies you believe that you need to be able to measure. If the fluctuations are much smaller than 10V (with certainty), then you wouldn't even need a voltage divider like you proposed, since the high 40VDC would be blocked and only the ripples would pass through.

 

[torquil]

You could make a small switching box that does this with selectable resistors. E.g. two switches that select between different pairs of resistors in the voltage divider, or with one resistor fixed. Then you'd be able to use your 10V oscilloscope for a wide range a voltage measurements!

Or buy one, I'm sure something like this exists.

 

[Bobbywhy]

Oh, shrinky! Have a look at these two sites for answers:

http://scope-probe-schematic.blogspot.com/

http://zone.ni.com/devzone/cda/ph/p/id/17

GOOGLE is your friend. If you learn to use it you will have access to TONS of information.